Fast response relay for firing ignitrons employing a phase shifter for controlling the relay



Nov. 9, 1965 Y. c. SUEL 3,217,238

FAST RESPONSE RELAY FOR FIRING IGNITRONS EMPLOYING A PHASE SHIFTER FOR CONTROLLING THE RELAY 8 SheetsSheet 1 Filed Nov. 3, 1961 Nov. 9, 1965 Y. c. SUEL 3,217,238

FAST RESPONSE RELAY FOR FIRING IGNITRONS EMPLOYING A PHASE SHIFTER FOR CONTROLLING THE RELAY Filed Nov. 3, 1961 8 Sheets-Sheet 2 S UARE I WAVE GEA/LPATORCC):

[HZ/Emir 7065 6 486, @Mh MM Nov. 9. 1965 Y. c. SUEL 3,217,238

FAST RESPONSE RELAY FOR FIRING IGNITRONS EMPLOYING A PHASE SHIFTER FOR CONTROLLING THE RELAY Filed Nov. 3. 1961 8 Sheets-Sheet I5 I 1*, I P I z I I I I t I [ZOE/3407,: I065 67 fad,

Nov. 9, 1965 Y. c. SUEL 3,217,238

FAST RESPONSE RELAY FOR FIRING IGNITRONS EMPLOYING A PHASE SHIFTER FOR CONTROLLING THE RELAY Filed Nov. 3, 1961 8 Sheets-Sheet 4 AUJMfiT/IBLE S/i/F TER LOAD C IPCU/ T 2 KM M Nov. 9, 1965 Y. c. SUEL 3,217,238

FAST RESPONSE RELAY FOR FIRING IGNITRONS EMPLOYING A PHASE SHIFTER FOR CONTROLLING THE RELAY Filed Nov. 3, 1961 8 Sheets-Sheet 5 Fig.6

Nov. 9, 1965 Y. c. SUEL 3,217,238

FAST RESPONSE RELAY FOR FIRING IGNITRONS EMPLOYING A PHASE SHIFTER FOR CONTROLLING THE RELAY Filed Nov. 3, 1961 8 Sheets-Sheet 6 Ph a5 52 I flflHrM h P'LZZ O swam/cm; 1! l 3 APPA ATUS ADJUSTABLE PHASE 8H/FTER X165 6 Fae/Z,

wwhywf Nov. 9, 1965 Y. c. SUEL FOR FIRING IGNITRONS EMPLOYING 3,217,238 A PHASE 8 Sheets-Sheet 7 FAST RESPONSE RELAY SHIFTER FOR CONTROLLING THE RELAY Filed Nov. 3, 1961 Nov. 9, 1965 SUEL 3,217,238

Y. C. FAST RESPONSE RELAY FOR FIRING IGNITRONS EMPLOYING A PHASE SHIFTER FOR CONTROLLING THE RELAY Filed Nov. 3, 1961 8 Sheets-Sheet 8 5QUARE WA VE EENEKATG he of" 72/65 6. 54565,

{9% M 44.4w 44m United States Patent 3,217,238 FAST RESPONSE RELAY FOR FIRING IGNITRONS EMPLOYING A PHASE SHIFTER FOR CON- TROLLING THE RELAY Yves Charles Sue], Thiais (Seine), France, assignor to Welding Research, Inc., Chicago, Ill., a corporation of Illinois Filed Nov. 3, 1961, Ser. No. 150,008 7 Claims. (Cl. 323-24) The invention relates to control circuits for electronic discharge valves having a firing electrode, and to various applications of said electric control circuits and has reference more particularly to such electric control circuits characteried by the use of a relay to which a periodic voltage is supplied, and wherein the said voltage is subjected to the action of an adjustable phase shifting device for thereby controlling the firing of the electric discharge valve.

Devices for firing electric discharge valves of the ignitron type which have a control electrode are well known and have been widely used in resistance welding machines, for example. These devices as first employed did not permit a precise regulation of the point of firing. To improve said regulation, electric valves such as thyratrons have been connected into the anode circuit and with the control electrode. With such an arrangement it is possible to control the value of the output current by regulating the firing of the thyratrons by acting on the phase of the voltage applied to the control grid of the thyratrons. However, thyratrons have known disadvantages such as a short life so that they require frequent replacing. Thyratrons also require a preheat time, and due to the power dissipated a reduction in the size of the cabinet in which the tubes are mounted is not possible. The purpose of the present invention is to remedy these disadvantages.

The main objective of the invention is to provide a control device for discharge valves having a control electrode, such as ignitrons, wherein a relay and its contact are placed in the circuit of the control electrode for controlling the passage of firing current to said electrode; a device characterized in that the relay for firing control is a rapid relay and is supplied by a device supplying a periodic electric voltage synchronous with the alternating voltage supplying the ignitrons or with a harmonic of this voltage combined with means to phase shift at will this periodic electric voltage with respect to the supply voltage of the ignitrons, which allows one to control the value of the voltage supplied by the ignitrons to the load circuit.

The invention extends also to applications of these control devices for discharge tubes with control electrode, to devices for supplying electric energy whose voltage is controlled by regulating the point of firing of the ignitrons and in particular to rectifiers and to the resistance welding machines.

In particular the invention is concerned with a rectifier to supply direct current from a polyphase supply, characterized in that the rectifying ignitrons in each phase are controlled respectively by a relay supplied by a periodic voltage produced from the corresponding phase of said relay, the phase of the supply which produce this periodic voltage being subjected to the action of an adjustable phase shifter which permits the control of the value of the rectified voltage which is produced.

The invention also deals with a resistance welding machine consisting of a welding transformer having welding electrodes in circuit with the secondary winding, said transformer being supplied by an alternating polyplrase power supply with the primary coils of the winding transformer being connected across each phase with the group of two ignitrons connected back-to-back, in series with 3,217,238 Patented Nov. 9, I965 each coil; a welding machine characterized in that the firing electrode of each group of ignitrons is controlled by the contact of a relay supplied by the phase of the polyphase supply corresponding to said group, the phases supplying said relays being subjected to the action of an adjustable phase shifter, which allow the control of the voltage and consequently the value of the welding current.

According to one way of utilization, a sequencing device displaces alternately by half a cycle the supply of the firing relays so as to render conductive at the recurring frequency of the sequencing device the ignitrons of each group which allows one to obtain finally a welding current formed by impulses alternately positive and negative, the length of each impulse corresponding to several cycles of the polyphase supply.

The invention deals also with a device to regulate the phase of the supply to the firing relay of the discharge tubes, which is characterized by a direct current supply connected to a potentiometer with center tap and to an alternating current supply for the firing relays connected to each relay through a rectifier and to the slider of the potentiometer so that the firing relay is actuated when the voltage of the alternating current source is larger than the voltage of the slider with respect to the center tap of the potentiometer, which allows one by moving said slider to regulate the moment of contact of the firing relay and hence to control the point of firing of the discharge tubes.

The invention also extends equally to characteristics described herewith and to their various combinations.

Devices for control of electric discharge tubes, such as ignitrons, and welding machines which conform to the invention, are shown as an example on the attached drawings in which:

FIGURE 1 is an electric circuit showing conventional thyratrons for firing back-to-back ignitrons;

FIGURE 2 represents schematically an improved control device as contemplated by the present invention for the firing of tWo ignitrons mounted back-to-back;

FIGURE 3 is a diagram on a time basis of the electric voltages and of the closure times of the control relay appearing in the device of FIGURE 2;

FIGURE 4 is a diagram on a time basis of the electric voltages appearing in the device of FIGURE 2 when the control relay is supplied by a sinusoidal voltage;

FIGURE 5 represents schematically a rectifier supplied by a three-phase alternating voltage;

FIGURE 6 is a diagram on a time basis of the various electric voltages appearing in the rectifying devices of FIGURE 5;

FIGURE 7 shows an electric circuit for a three phase resistance welding machine;

FIGURE 8 shows on a time basis the form of the Welding current;

FIGURE 9 represents a control device for controlling the phase of the supply to the firing relay;

FIGURE 10 represents on a time basis the voltages appearing in the device of FIGURE 9; and

FIGURE 11 represents the device of FIGURE 2 with the addition of a protective device for the ignitrons.

FIGURE 1 represents a known circuit wherein electric discharge valves having a control electrode, such as ignitrons 10 and 11, are connected in bacl to-back relation. The same is provided with alternating current from the source 13 and thus the power device will control the current delivered to the secondary of the load transformer 12. For firing the ignitrons l0 and 11 a pair of thyratrons 31 and 32 are employed. The thyratrons alternately control the conductance of the ignitrons by passing anode current to the control electrode 14 of either ignitron 10 or ignitron 11. In this circuit arrangement it will be understood that the value of the primary current can be controlled by regulating the firing of the thyratrons 'by acting on the phase of the current which supplies the primaries of the peak transformers 33 and 34. The said peak transformers control the voltage, indicated by numerals 41 and 42, on the control grid of the thyratrons. The device of FIGURE 1 is not entirely satisfactory since it has all of the disadvantages due to the use of thyratrons as previously explained. The control circuits of the invention eliminate the use of thyratrons by substituting therefor a fast acting relay and contactor as basically disclosed in FIGURE 2, which will now be described.

The control device for the ignitron tubes represented in FIGURE 2 in the case of two ignitrons 10 and 11 connected back-to-back consists of a circuit 3 producing a square wave 26, see diagram of FIGURE 3, at a frequency double the frequency of the industrial current supplied by the alternating current source 13, this circuit 3 supplies a rapid relay 2 which controls the firing of the ignitrons whose contact 1 controls the circuit of the control electrodes 14 of the ignitrons 10 and 11, so that the main current is established in the load transformer 12 when contact 1 is closed.

To control the value of the primary current, it is sufficient to control the point of closure of contact 1 with respect to the voltage of the single phase alternating supply 13. It is known that if one advances the phase, the effective value of the current increases and vice versa. To do this, one acts on the phase of the square wave 26 by the action of a phase shifting circuit 16 which controls the phase of the supply 13 to circuit 3 which produces the square wave 26.

FIGURE 3 represents the diagram on a time basis of the various voltages which appear in the operation of the control device for the closure of relay 2 which controls the firing of the ignitrons. The voltage of supply 13 is represented by the sine wave 24. The phase of the square wave 26 with respect to this sine wave 24 is represented by length 21, the closing time of relay 2 before closure of contact 1 is shown by length 22. The delay in the closure of contact 1 with respect to voltage 24 is represented by length 2% which is also equal to the sum of lengths 21 and 22. The firing of the ignitron takes place at the time 23 and it is at this moment that the main current 25 is established.

It is known that immediately after the firing of an ignitron, contact 1 can be opened. This is why the current can be disconnected in relay 2 which controls the firing of the ignitrons immediately after time 23. The width of the square wave 26 must then be somewhat greater than time 22. Finally the contact is opened at 27. With these elementary conditions established, it is seen that operation at 120 cycles is possible only if the make time plus the break time is equal to or less than about 8 milliseconds.

One can also, according to the invention, use a sinusoidal current in place of a square wave to operate on the relay 2 which controls the firing of the ignitrons. FIGURE 4 shows, as a function of time, a supply sine wave 50, a phase shifted voltage 51, by an amount 52 with respect to 50. The relay 2 is supplied directly by voltage 51 which can be phase shifted more or less by adjusting the phase shift 52. The curve 53, which is obtained by squaring the values of voltage 51 is proportional to the force of attraction of the relay armature, and has a frequency of 120 cycles. The contact of relay 2 is closed according to the curve 54. The delay with respect to the zero of curve 53 is the amount 55. It is a characteristic of the relay. Accordingly the primary current is established according to curve 56.

The same result can also be obtained with relay 2 operating with direct current, supplied from an alternating current which is full wave rectified. Finally, if one uses relays having make time of the order of several milliseconds, a very great precision of operation is obtained by supplying said relays with a transformer producing in the secondary peak voltages in the form of very narrow square waves of the order of several milliseconds.

Thus, the total of these arrangements allows one to eliminate the use of thyratrons, and consequently the disadvantages indicated heretofore resulting from their use.

The invention applies not only to the devices for the firing of electric discharge tubes, as described heretofore, but equally to various applications of these devices, particularly to the installation for the controlled supply of electric energy to load circuits such as electric motors, welding machine electrodes, etc.

FIGURE 5 represents a rectifier with ignitrons in which the direct current output voltage is regulated by delaying the firing of the ignitrons. The three-phase alternating supply is applied to a supply transformer 61 in which each secondary coil 68 68 and 68 corresponding to each of the phases of the supply is connected to a corresponding ignitron 62 62 and 62 The circuit of the firing electrode 14 of each ignitron 62 containing a resistance 67 67 and 67 for limiting the firing current, is closed by contacts 63 63 and 63 controlled respectively by the relays 66 66 and 66 which control the firing. These relays 66 66 and 66 are supplied respectively through an adjustable phase shifter 60 and the rectifiers 65 65 and 65 from each phase of the supply voltage. The ignitrons 62 62 and 62 thus connected as a rectifier have their output on a load circuit 64.

The rectifier described above operates in the following manner: The voltages of the three phases of the three-phase supply are represented on a time basis on the diagram of FIGURE 6 by sine waves 71, 72 and '73 which are displaced by 120 degrees. The make time of contacts 63 63 and 63 are represented respectively as a function of time on FIGURE 6 by curves 74, 75 and 76. The main current flowing through the load circuit 64 is represented for each phase by the curves as follows: For curve 77 which corresponds to the sine wave 71 and to the closure of the contact 63 according to the curve 74. For curve 78 which corresponds to the sine wave 72 and to the closure of the contact 63 according to the curve 75. For curve 79 which corresponds to the sine wave 73 and to the closure of the contact 63 according to the curve 76.

To modify the load voltage, it is necessary to modify the phase of the supply of relays 66 66 and 66 by operating on the phase shifter 60.

FIGURE 7 represents an application to a resistance welding machine of the control device firing the ignition tubes. I11 such a welding machine the alternating current of the supply such as a three-phase current, must be transformed in successive impulses of current of an adjustable length containing several cycles of the supply current and sufliciently long to eliminate the influence of the inductance of the arms of the machine.

The welding machine shown in FIGURE 7 consists of a welding transformer with a primary formed by three coils 83, 84 and 85 connected respectively to each of the phases Ph1, PM and Ph? of the three-phase supply and to a single secondary connected to the welding electrodes. The three primary coils 83, 84 and 85 are placed on the same core of a single-phase transformer and are strongly coupled to the secondary. They are connected between the phases Phll (terminal 80), Ph2 (terminal 81), and Ph3, (terminal 82) with a series connection for each of the primary coils of two ignitrons 86 and 87 connected back-to-back. The firing of each group of ignitrons 86 and 87 is controlled respectively by contacts 88 88 and 88 of the relays 89 89 and 89 according to the arrangements of FIGURE 2.

The welding machine described above operates in the following manner: By referring to the diagram of FIG- URE 8, which represents the secondary current consisting of impulses 100 100 etc., successively of opposite directions, each of which consists of several cycles of the supply current. One accepts that when the current flows in each primary coil in the positive direction, the induced current in the secondary is positive and vice versa. In order for the current to be positive, the three ignitrons 86 must become conductive successively. To obtain a current flowing in the opposite direction, the three ignitrons 87 must become conductive.

This result is obtained in the following manner: The firing of one of the ignitrons 86 or 87 of each phase is caused by the closure of contact 88 in the same manner as in the diagram of FIGURE 2. Relays 89 89 and 89 corresponding to each of the contacts 88 88 and 88 are supplied from a three-phase source through a phase shifter 90, and the current is rectified by the diodes 91 and 92. A reversing switch 93 is controlled by the coil 94 through sequencing apparatus 96. According to the position of contact 93 coils 89 are excited once every cycle, either when the diodes 91 become conductive or through a phase shift of a half cycle when diodes 92 become conductive. In the first case, ignitrons 86 are conductive and the secondary current is positive, (impulses 100 100 etc.) In the second case, ignitrons 87 are conductive and produce a secondary current which is a negative (impulses 100 100 etc.).

It is possible to produce several modifications of the devices described above without going out of the range of the invention. In particular, one can bring about an interesting improvement which interrupts the current in the machine in case of damage. This protective device is applicable to the various arrangements which have been examined, but to simplify its description, it is added to the simplest case, that of FIGURE 2. It contains, see FIGURE 11, a contact 4 connected in series in the firing circuit of the ignitrons and which interrupts their operation when it is open. This contact 4 is controlled by the protective relay 5 so that it remains closed whenever the protective relay 5 is not excited. Relay 5 which is a slow relay is excited by means of a supply circuit consisting of a transformer 7 and a resistance 8. With relay 2 at rest, the contact 6 of that relay short circuits the coil of relay 5. In order to excite relay 5, it is necessary for relay 2 to be excited and its contact 6 open.

When the relay 2, controlling the firing, is excited under the impulse current supplied by device 3, the protective relay 5 receives'successive impulses of current from transformer 7 but these impulses are narrow with respect to the inertia of the protective relay 5 and the armature of this relay remains stationary so that the control firing of the ignitrons 10 and 11 takes place normally.

If damage occurs, in particular, if relay 2, controlling the firing remains stuck, contact 1 is closed continuously, contact 6 is opened, and the protective relay 5 is excited. Its contact 4 opens the firing circuit of ignitrons 10 and 11. Consequently, ignitrons 10 and 11 do not continue firing. The device above insures a protection for the electric discharge tubes, this protection being of greater interest since when using thyratrons for the control of discharge tubes, this advantage is not easily obtained.

Another interesting improvement consists of using rapid relays of a special type for the control firing. These are relays in which the contacts are made by metallic pieces on which adheres a drop of mercury. This arrangement results in the forming of a bridge between the contacts of the normally closed and normally open contacts during the closure of the relay. The duration of this connection is very short but sufficient to allow the firing of the ignitrons.

Thus, it is seen that by supplying the coil of the relay controlling the firing by a current of appropriate shape, a square wave for example, and at a frequency of 60 cycles, two connections per cycle will be obtained which will give the firing of the two ignitrons 10 and 11. If following a mechanical or electrical accident the relay does not operate any longer, the temporary connection between the two contacts does not take place and the current of the ignitrons is thus interrupted. This device consists by itself in a self-protecting device.

Finally it is known that in particular to improve the quality of welding in resistance welding machines, one begins sometimes by applying to the parts to be welded a preheating current. After welding, it is postheating current which completes the operation. It is also necessary to vary the welding current in a progressive manner. In the case of a rectifier supplying a motor, it is necessary to regulate its speed in a continuous manner.

In order to resolve these various problems, one can use the explanation given below as an example. In order to simplify the explanation, a half-wave three-phase rectifier has been chosen which is the simplest case. This is the drawing of FIGURE 5.

Each of the relays 66 66 and 66 is connected to a device whose drawing is shown on FIGURE 9. A direct current supply 99 is' connected to a potentiometer with center tap 101. Transformer 102 supplies the circuit with a current 105 of suitable phase. Relay 66 is a fast relay, and diodes 104 allow the passage of current in the relay only when the voltage of the transformer 102 is greater than the voltage of the slider with respect to the center tap of potentiometer 101.

On the diagram of FIGURE 10 the slider 103 has been chosen to have two different positions: In posit-ion (l) the slider 103 is positive with respect to the center tap. The difference of potential is 106. At time 107 current will flow through relay 66. After a certain delay, its contact 63 will close. In position (2) the slider is negative. The voltage difference is 108. At time 109, relay 66 is excited, and after a certain delay, contact 63 is closed. It is seen that by varying only the applied voltage, the firing point of the ignitron has been phase shifted.

Potentiometer 101 and its slider 103 are only one way to obtain a variable voltage. It is evident that all the resources of electronics can be used to produce control voltages allowing the use of this principle in numerous applications. It is possible, for example, to use a relay 66 with two coils, one of the coils being supplied by the alternating voltage furnished by transformer 102, the other by the direct current voltage furnished by potentiometer 101. These two supplies are then isolated.

What is claimed is:

1. In an electric circuit including an alternating current source connected to a load and wherein an electric dis-charge valve passes said current to the load when the valveis rendered conductive, of a control electrode for said discharge valve for rendering the valve conductive when the control electrode is energized, a contactor in the circuit of the control electrode, a fast acting relay for actuating the contactor to close the circuit to the electrode when the relay is energized, means supplying a periodic voltage to the relay for energizing the relay and which is synchronous with the voltage of the alternating current source connected to the load or with a harmonic of said voltage, and additional means for phase shifting the periodic voltage with respect to the voltage of said alternating current source, whereby the energization of the relay and thus the point of firing of the valve with respect to the voltage of the alternating current source can be regulated to control the magnitude of the current supplied to the load.

2. In an electric circuit as defined by claim 1, wherein the periodic voltage supplied to the relay is a square wave and wherein the time of each pulse is at least equal to the make time of the relay.

3. In an electric circuit as defined by claim 1, wherein the periodic voltage supplied to the relay is a sinusoidal Voltage.

4. In a power circuit including a source of poly-phase alternating current, an inductive drive having primary windings corresponding in number to the phases of the alternating current source, means connecting each primary winding to its respective phase of said source, at least one electric discharge valve for each primary winding for controlling flow of current through the winding, a control electrode for each discharge valve, a contactor in the circuit of each control electrode, a relay for actuating each contactor, means supplying a periodic voltage to each relay and which is produced by the phase corresponding to said relay, and additional means acting on the phases of the alternating current source for phase shifting the periodic voltages, whereby the point of firing of the electric discharge valves with respect to the voltage of the alternating current source can be regulated to control the magnitude of the rectified current permitted to flow through the windings.

5. In a power circuit including a source of polyphase alternating current, an inductive device having a secondary winding and having primary windings corresponding in number to the phases of the alternating current source, means connecting each primary winding to its respective phase of said source, a pair of electric discharge valves connected in circuit with each winding and in back-to-back relation, whereby one group of valves passes unidirectional current impulses in one direction and the other group passes unidirectional current impulses in the opposite direction, a contactor for each pair of valves having connection with the control electrode of said valves, a relay for each contactor for actuating the same, means for energizing each relay including a periodic voltage which is produced by the phase corresponding to the respective relay, and additional means acting on the phases of the alternating current source for phase shifting the periodic voltages, whereby the point of firing of the electric discharge valves can be regulated to control the magnitude of the rectified current produced in the secondary winding.

6. -In an electric circuit including an alternating current source connected to a load and wherein an electric discharge valve passes said current to the load when the valve is rendered conductive, of a control electrode for said discharge valve for rendering the valve conductive when the control electrode is energized, a contactor in the circuit of the control electrode, a relay for actuating the contactor to close the circuit to the electrodes when the relay is energized, means supplying a periodic voltage to the relay for energizing the relay and which is synchronous with the voltage of the alternating current source connected to the load or with a harmonic of said voltage, additional means for phase shifting the periodic voltage with respect to the voltage of said alternating current source, whereby the energization of the relay and thus the point of firing of the valve with respect to the voltage of the alternating current source can be 'regulated to contol the magnitude of the current supplied to the load, said phase shifting means including a direct current supply connected to a potentiometer having a center tap, and said alternating current source having one terminal thereof connected to the center tap and having its other terminal connected to the potentiometer by and adjustable tap with the said relay and a rectifier in advance of the relay being included in the circuit, whereby the relay is energized only when the voltage of the alternating current source is greater than the voltage of the adjustable tap with respect to the center tap.

7. In a power circuit including a source of polyphase alternating current, an inductive device having a secondary winding and having primary windings corresponding in number to the phases of the alternating current source, means connecting each primary winding to its respective phase of said source, a pair of electric discharge valves connected in circuit with each winding and in back-to-back relation, whereby one group of valves passes unidirectional current impulses in one direction and the other group passes unidirectional current impulses in the opposite direction, a contactor for each pair of valves having connection with the control electrode of said valves, a relay for each contactor for actuating the same to close the circuit to the electrodes when the relay is energized, means for energizing each relay including a periodic voltage which is produced by the phase corresponding to the respective relay, additional means acting on the phases of the alternating current source for phase shifting the periodic voltages, whereby the point of firing of the electric discharge valves can be regulated to control the magnitude of the rectified current produced in the secondary winding, circuit means connecting the relays to their respective source of phase shifted voltage, the said circuit means including a pair of rectifiers for each relay in back-to-back relation, a switch in electrical association with the terminals of each pair of rectifiers, a sequencing device, and connections between the sequencing device and the said switches, whereby said switches in one position will connect the relays for receiving the phase shifted positive half cycles of the periodic voltages, respectively, and whereby said switches in their other position will connect the relays for receiving the phase shifted negative half cycles, respectively, of said periodic voltages.

References ited by the Examiner UNITED STATES PATENTS 2,340,076 1/44 Pearson et al. 328-8l 2,361,845 10/44 Hutchins 323- 24 2,431,284 11/47 Stadum 323-34 2,703,860 3/55 Large et al. 323-24 2,840,776 6/58 Van Ness 32323 X 2,951,197 8/60 Broyles et al. 323-34 LLOYD MCCOLLUM, Primary Examiner. 

7. IN A POWER CIRCUIT INCLUDING A SOURCE OF POLYPHASE ALTERNATING CURRENT, AN INDUCTIVE DEVICE HAVING A SECONDARY WINDING AND HAVING PRIMARY WINDINGS CORRESPONDING IN NUMBER TO THE PHASES OF THE ALTERNATING CURRENT SOURCE, MEANS CONNECTING EACH PRIMARY WINDING TO ITS RESPECTIVE PHASE OF SAID SOURCE, A PAIR OF ELECTRIC DISCHARGE VALVES CONNECTED IN CIRCUIT WITH EACH WINDING AND IN BACK-TO-BACK RELATION, WHEREBY ONE GROUP OF VALVES PASSES UNINDIRECTIONAL CURRENT IMPULSES IN ONE DIRECTION AND THE OTHER GROUP PASSES UNIDIRECTIONAL CURRENT IMPULSES IN THE OPPOSITE DIRECTION, A CONTACTOR FOR EACH PAIR OF VALVES HAVING CONNECTION WITH THE CONTROL ELECTRODE OF SAID VALVES, A RELAY FOR EACH CONTACTOR FOR ACTUATING THE SAME TO CLOSE THE CIRCUIT TO THE ELECTRODES WHEN THE RELAY IS ENERGIZED, MEANS FOR ENERGIZING EACH RELAY INCLUDING A PERIODIC VOLTAGE WHICH IS PRODUCED BY THE PHASE CORRESPONDING TO THE RESPECTIVE RELAY, ADDITIONAL MEANS ACTING ON THE PHASES OF THE ALTERNATING CURRENT SOURCE FOR PHASE SHIFTING THE PERIODIC VOLTAGES, WHEREBY THE POINT OF FIRING OF 